1. Field of the Invention
The present invention is directed to improved methods for directly electroplating conducting metals onto nonconducting or dielectric base materials particularly those containing acrylic or epoxy compounds subject to swelling under traditional plating methodologies. More particularly, the present invention is directed to reduced cost, reduced toxicity, streamlined methods for directly electroplating metals onto the surfaces of nonconducting substrates without the need for preliminary electroless plating, strongly oxidizing or exotic adhesion promoters, exotic activation catalysts, conversion coatings, or conducting clips to initiate the propagation of plating metal. The methods of the present invention are particularly applicable to rigid/flex sandwich materials incorporating acrylic adhesives, epoxy matrix multi-wire circuits, multi-layer laminated circuit boards, copper clad substrates, molded circuitry, and non-clad substrates. Additionally, the methods of the present invention can be readily incorporated into pattern plating processes.
2. Description of the Prior Art
In the past decade a number of methods have been developed for "directly" electroplating metals onto the surfaces of non-metallic or dielectric substrates. Prior to these developments, non-metallic surfaces were typically metallized or plated by first making the surfaces catalytically receptive to electroless metal deposition by treatment with an activation catalyst (usually an acid suspension of palladium and tin salts) followed by treatment of the catalyzed surfaces in chemical plating baths. These electroless or chemical plating steps form a metal layer having a sufficient thickness to support subsequent electroplating steps which build a layer of electroplated metal having the desired thickness and conductivity. Unfortunately, these traditional multi-step electroless metal plating processes require careful monitoring by highly trained operators because of their use of formaldehyde, complexity and sensitivity to chemical changes in the various solutions and plating baths. In spite of this complexity and expense early electroless plating processes continue to be of use, particularly in the manufacture of printed circuit boards.
Subsequent efforts directed toward eliminating the complexity and expense associated with preliminary electroless plating steps have met with varied levels of success. For example, Morrissey et al., U.S. Pat. No. 4,683,036 report a method for directly electroplating non-metallic surfaces by forming a plurality of metallic sites on the surface to be plated, providing that surface with a connector electrode, exposing the metallized surface and at least a portion of the connector electrode to an electroplating bath incorporating plating metal and a variety of preferential deposition additives including dyes, surfactants, chelating agents, brighteners and leveling agents and applying an electric potential to the connector electrode and a counter electrode in the plating bath.
Though reportedly effective at the direct electroplating of metals onto nonconducting surfaces, the method of Morrissey et al. also requires its own careful monitoring and control of plating bath chemistry and plating potential in order to obtain uniform metallization. Moreover, to compensate for the lack of preliminary electroless metal layer Morrissey et al. rely on a wide variety of relatively toxic plating additives to achieve preferential metal deposition in their plating baths. These materials add to the complexity, toxicity and waste elimination expenses associated with such processes. Further, because the catalytically deposited discrete metallic sites are not sufficiently conductive to support direct electroplating on their own a conducting "connector electrode" is required in conjunction with the metallized surface to initiate plating.
Morrissey et al. also report it advantageous to further treat the catalytically activated surface with an acidic or strongly basic solution in order to remove tin from the treated surface prior to electroplating. As with earlier electroless plating methodologies, these acidic or strongly basic accelerating solutions add to the expense, toxicity, and disposal problems associated with these plating techniques.
In an analogous development, recent efforts at improving the speed and quality of metallic plating processes have utilized adhesion promoter solutions to enhance the catalytic activation of dielectric surfaces prior to chemical or electroplating. Typically, these adhesion promoters are formed of aqueous solutions containing compounds such as potassium permanganate. These solutions oxidize the substrate surfaces in a preliminary treatment step prior to catalytic activation with the precious metal/tin catalyst. Though successful for these purposes, as with the previously discussed accelerating solutions, these strongly oxidizing adhesion promoters are difficult to utilize and dispose of in a safe and environmentally conscious manner.
Additionally, many of the prior art adhesion promoter solutions and condition/cleaners chemically interact with acrylic or epoxy materials causing excessive swelling. Swelling of the substrate reduces catalyst adhesion and significantly degrades plating performance. For example, modern flexible circuit boards are now formed by sandwiching layers of flexible material, such as Kapton.TM., with layers of rigid material, such as FR-4 or tetra-functional polyamide. Layers of these materials are attached to one another with an acrylic adhesive. However, this adhesive is subject to swelling and attack by the combination of known saline adhesion promoter solutions followed by treatment with the typical nonylphenol metallized cleaners. Similarly, modern multi-wire circuits are formed by laying insulated copper conductors in a matrix of epoxy. These materials also are subject to excessive swelling of the epoxy matrix in areas where the insulation has been removed to expose the copper conductor. Thus, modern direct plating technologies are of limited utility with these materials.
Accordingly, it is a principal object of the present invention to provide methods for directly electroplating dielectric substrates with high quality, uniform, conducting metal layers that are simple to operate, inexpensive, and relatively non-toxic. The methods of the present invention are particularly applicable to the production of flexible circuitry, multi-wire circuitry, acrylic and/or epoxy containing substrates, and printed circuit boards having copper clad conductive surfaces on opposite sides of an insulating dielectric substrate and connected by plated "through holes".
It is an additional object of the present invention to provide novel, low cost adhesion promoters that are non-hazardous, highly effective, and inexpensive to produce.
It is a further object of the present invention to provide methods for the direct electroplating of non-conducting substrates that are particularly well suited for utilization in modern pattern plating processes and, of equal importance, operate without the need for high plating current densities and correspondingly large "through hole" diameters.
It is a further additional object of the present invention to provide high quality treated dielectric substrates and directly plated dielectric substrates including rigid/flex materials and acrylic or epoxy containing materials.